1: Don't
2: There are good cnc machines and cheap cnc machines. There are no good and cheap cnc machines.
3: What may help if you are very, very technical, both on the software side and the mechanical side.
4: If you are good with linux, linuxcnc can probably tie all the old hardware together that has no value for anyone else and thus is cheap.
But.....
even a good small cnc for 2000.-- will be hard, not to mention the size you mention.
Brain wave:
You do not mention what you are going to do with it nor where you are located.
You may get far if you do not need super precise results with steel i-beams, rollerskate wheels and "bicycle chain and sprockets"
But it is not going to be easy...!